function imgSwap(oImg) {
   var strOver  = "-on.jpg";
   var strOff = ".jpg";
   var strImg = oImg.src;
   if (strImg.indexOf(strOver) != -1) 
      oImg.src = strImg.replace(strOver,strOff);
   else
      oImg.src = strImg.replace(strOff,strOver);
}